"Certainly one of the chief guarantees of freedom under any government, no matter how popular and respected, is the right of citizens to keep and bear arms. This is not to say that firearms should not be very carefully used, and that definite safety rules of precaution should not be taught and enforced. But the right of citizens to bear arms is just one more guarantee against arbitrary government, one more safeguard against tyranny which now appears remote in America, but which historically has proved to be always possible"